The present invention relates to a technique useful in a mobile radio communication device and further to a super high-frequency band mobile radio communication device using a GaAs FET (gallium arsenide field effect transistor) in a high-frequency power amplifier, for example, in a digital mobile telephone or a cordless telephone.
FIG. 2 shows an outline constitution of a mobile radio communication device which was investigated by the present inventors prior to the present invention.
A mobile radio communication device shown in FIG. 2 is intended to be used especially for a mobile telephone whose main parts are composed of a signal source unit (base band unit) 1 generating sending signals to be signalled on the radio frequency, a sending unit (radio-frequency (RF) power module) 2 amplifying a sending signal from the signal source unit 1 to a predetermined sending power using GaAs FET 20, a receiving unit 3 carrying out the reception and demodulation of a radio signal, an antenna connecting unit 4 connecting output of the sending unit 2 and input of the receiving unit 3 to a common antenna unit 41 in the separated condition, an operation unit 5 gathering operating functions such as key switches and indicators, a power supply unit 6 supplying the positive operating source (+Vdd) being derived from the built battery to every part in the unit, a control unit 7 controlling centrally each unit (1-6) by integrated circuit universal logic control units, so-called micro computers (CPUs), a microphone 8 and a loudspeaker 9 carrying out sending and receiving speeches.
In the mobile radio communication device, (1) high level or multiple functioning has been made possible by the logic control of microcomputers. Also (2) the use of radio waves of super high frequency band such as 1.5 GHz band has been made possible by using a GaAs FET 20 for high-frequency power amplification in the sending unit 2. Further (3) the main functions, such as sending and control, are facilitated by single units thereby increasing the efficiency of the package and the degree of flexibility of the overall design.
By the above-mentioned items (1), (2) and (3), for example, a mobile telephone capable of being linked with a general public telephone line can be of a smaller type.
Here, in the above-mentioned device, when a radio signal is RF-power amplified using the GaAs FET 20, in the RF power amplifier, i.e., final amplifier 24, in addition to the positive operating supply voltage +Vdd, the negative bias voltage -VB is required so as to render the GaAs FET 20 inoperative. In order to obtain the negative bias voltage -VB, a communication device in the prior art has used an inverter power supply being composed of a combination of an independent oscillator and a rectifier. However, a problem is found in that when the inverter power supplies are built in the radio communication devices, the outputs of oscillators of those inverter power supplies become noise sources interfering with the normal operation of the radio communication devices.
Further, the inventors of the present invention have investigated that in the sending unit 2 for RF-power amplifying radio signals as shown in FIG. 2, a part of the radio signals outputted from a buffer amplifier 21 of the former stage is branched by a coupler 25, and the branched radio signals are rectified and smoothed by diodes and capacitors, thereby the negative bias voltage -VB is supplied to the final amplifier 24. That is to say, the negative voltage generator 26, with radio signals being made the input power supply, is built in the sending unit 2 (see JP-A--63-202106).
Thereby super high-frequency radio transmission can be carried out by a high-frequency power amplifier requiring a negative bias voltage -VB even if the negative voltage is not supplied from the outside.
However, the inventors of the present invention have revealed that the following problems exist in the above technique.
That is, (1) if radio signals are linear signals, the system in FIG. 2 is effective, as the energy of the radio signals can be deemed constant by taking the average thereof. But, if the radio signals are digital signals, the energy of the radio signals is not always constant on account of the modulation or the sending stop at the time of standby. For example, in the case of a digital mobile telephone, radio signals outputted from the base band unit 1 are made burst signals. Therefore, the energy of the signals in themselves varies according to whether sending signals exist or not. Therefore, stable and accurate negative bias voltage -VB cannot be obtained from the negative voltage generator 26 having radio signals being made burst signals as the input source. As a result, the case may occur in which stabilizing of the sending operation becomes difficult.
(2) Generally speaking, when an AC signal of the same power is obtained, the power consumption and the cost of the circuit necessary to get the AC signal tend to increase as the frequency of the AC signal becomes higher. Therefore, use of especially super high frequency radio signals such as 1.5 GHz in order to produce the DC negative bias voltage --VB would cause already limited power sources to be excessively consumed and circuit costs to be increased more than necessary.
(3) When super high frequency radio signals being not less than UHF or microwave bands are dealt with, the main circuits in the sending unit 2 are composed of a distributed constant such as slip lines. Therefore, the negative voltage generator 26 may be required to be installed with sufficient buffer spaces not to interfere with the distributed constant. As a result, necessary dimensions of the sending unit 2 are more significantly increased than required fundamentally by the negative voltage generator 26, and the efficiency of the package and the flexibility in external shape design may be significantly damaged.
(4) As the generation of the negative bias voltage -VB is carried out only inside the sending unit 2, when the negative bias voltage -VB is to be controlled centrally to the external control unit 7, for example, for switching the transmission power, the constitution would be complicated because of an increase in the wiring between units.